In 2011, our plant was selected to participate in a special study with the EPA and the West Virginia Department of Health and Human Resources’ Office of Environmental Health Services (WVDHHR/OEHS). This special study was specifically created to research and find solutions for DBP removal. Over two years, the study group met quarterly and was very beneficial to me and our water system.
Our first step was to make some treatment changes in the plant. I started with the reduction of pre-chlorine feed, the addition of a carbon feed system, and optimization of the potassium permanganate feed. These minor changes alone made a sizable impact on our TTHM levels, but we still had issues in the warmer months.
While participating in the study and through research and field testing, we found that aeration was the best way to remove TTHMs after they are formed. In fact, many systems have been installing aeration systems in the distribution tanks. This led me to my next step: I would install one aeration system in the clearwell of our plant and - with an initial start-up cost of $3,500 and an annual cost of $600 - save the system money. My reasoning was that if I could reduce TTHMs in the plant by putting an aeration system in the clearwell, it would in turn lead to a reduction in the distribution system.
I chose to use a spray system. This type of system uses special spray nozzles that disperse the water in tiny beads, which strip the TTHMs from the water. At the same time, this mixes the clearwell and turns it over, also reducing TTHMs. I installed two spray nozzles in the clearwell to cover the whole area of the water. The pump used for spraying is wired to a timer that runs on three-hour intervals throughout the day. Given the size and flow of the pump, this timing turns over our clearwell twice in three hours. This optimizes the TTHM removal process by insuring maximum turnover rates and spray time.
After the system was installed, I conducted my own special study to determine what removal rate was being achieved. Using the Hach DR3900 and the Hach THM Plus Method, keeping all other variables constant, I ran two tests every morning for a week with the aeration system completely shut off. The next week, keeping all other variables constant, I ran two tests every morning with the aeration system running (see Fig. 2). These results, along with required compliance testing results, showed removal rates exceeding 40 percent. My original hypothesis was correct in that by removing TTHMs in the plant, it also reduced them in the system. That quarter’s compliance testing results were the lowest we had ever seen in over seven years of testing.
